Vehicle power units having a conventional balancer mechanism include a type in which a drive gear provided on a crank shaft and a driven gear on a balancer shaft directly engage with each other and transmit rotary driving force from the crank shaft to the balancer shaft (see Patent Document 1).
This type of power units have a problem in that backlash noise occurs between the drive gear and the driven gear arranged between the crank shaft and the balancer shaft when fluctuation occurs in the rotation of the crank shaft in a state with increased backlash due to temperature variations accompanying the operation of the power unit. Further, in cases of using layout structure with a long interval between the crank shaft and the balancer shaft, problems arise in that the drive gear and the driven gear are required to increase in diameter and weight. This causes the crankcase to be enlarged outward, and the power unit to be increased in size.